In modern computed tomography (CT) scanner systems, an X-ray source generates an X-ray beam which interrogates an object and is incident on a sensor array. In third-generation CT systems, the source and sensor array are mounted on a gantry which rotates about the object. Successive sets of projections of the object are recorded at incremental gantry rotation angles. After completion of a half rotation of the gantry ("half-scan" system) or a full rotation ("full-scan" system), data from the successive rotation angles are combined in a process known as reconstruction to create a cross-sectional image of the object. In a stationary scan configuration, the object is fixed in position during each scan, while in a translational scan, or "helical" scan, the object translates relative to the gantry during a scan, improving system throughput, but complicating image reconstruction.
In a conventional two-dimensional CT scanner of the third generation type, the X-ray beam propagates in a planar fan shape between a point source and a sensor array comprising a one-dimensional array of detector elements. The fan beam is referred to as a "transaxial fan" because the plane of the fan is perpendicular to the rotation axis, i.e., the z-axis. A two-dimensional image reconstruction process collects the raw data at each rotation angle and following a half-scan, or full-scan, converts the data into a planar pixel image of the portion of the object through which the X-rays have passed. Following each scan, the object may be translated along the z-axis to generate adjacent planar cross-sectional images or "slices" of the object which can be combined to produce a volumetric image.
To speed up volumetric imaging of the object, a three-dimensional CT scanner employs a conical X-ray beam, also referred to as "cone beam", generated at a point source, which projects through the object and is incident on a two-dimensional sensor array. The array typically comprises multiple rows and multiple columns of detectors which lie on a cylindrical surface. In this configuration, the X-ray cone beam diverges not only along the plane of the transaxial fan, but also diverges along the z-axis.
In practice, a conventional two-dimensional reconstruction algorithm is insufficient for reconstructing a three-dimensional volumetric image from cone-beam data collected with a two-dimensional detector array. The three-dimensional cone-beam data cannot be accurately resolved into independent parallel layers along the z-axis for introduction into two-dimensional reconstruction since each transaxial fan (defined as the portion of a conical beam passing through a corresponding row of detectors) lies at a conical angle relative to the z-axis that varies from one detector to the next. Performing two-dimensional reconstruction using this data would therefore result in reconstruction errors for each set of fan beam data, with the exception of the central transaxial fan along the xy-plane (where the plane of the transaxial fan is normal to the z-axis). The reconstruction errors worsen as the conical angle increases from the zero angle defined by the central transaxial fan. A more accurate three-dimensional reconstruction technique known as cone-beam reconstruction for stationary scan configurations is described in:
1. L. A. Feldkamp, L. C. Davis, and J. W. Kress, "Practical Cone-beam Algorithm", J. Opt. Soc. Am. A, Vol.1, p612, No.6, June 1984. PA1 2. U.S. Pat. No. 5,291,402 issued Mar. 1, 1994, to A. H. Pfoh, "Helical Scanning Computed Tomography Apparatus"; PA1 3. U.S. Pat. No. 5,377,250 issued Dec. 27, 1994, to H. Hu,"Reconstruction Method for Helical Scanning Computed Tomography Apparatus with Multi-row Detector Array"; and PA1 4. U.S. Pat. No. 5,430,783 issued Jul. 4, 1995, to H. Hu, N. J. Pele, and A. H. Pfoh, "Reconstruction Method for Helical Scanning Computed Tomography Apparatus with Multi-row Detector Array Employing Overlapping Beams".
The foregoing discussion applies to scanning an object which is stationary with respect to the z-axis. In another form of scanning, known in the art as a helical scan, the object translates relative to the gantry along a translation axis, usually parallel to the z-axis, at a constant speed during gantry rotation. From the perspective of the object, the x-ray source and sensors can be envisioned as circling about the object in a helical trajectory during data collection. In a helical scan performed by a conventional system having a single row of detectors, the projection data are first interpolated to the z position of each slice for generating its planar image. These planar images are located contiguously along the z-axis. The contiguous slices can be combined and further processed for various modes of three-dimensional display. Unfortunately, in a cone-beam system, the z-axis translation causes the collected data to deviate further from that data which is required for standard two-dimensional or three-dimensional reconstruction techniques. As a result, the reconstruction errors arising from a helical scan using a cone-beam system are worse than that of a stationary scan. Reconstruction and enhancement methods for cone-beam helical scans are described in: